Posted
by
Zonk
on Sunday January 07, 2007 @05:59PM
from the do-not-go-here dept.

dido writes "The BBC reports that the Hubble Space Telescope has been used to make a map of the dark matter distribution of the universe, providing the best evidence of the role dark matter plays in the structure and evolution of the universe. From the article: 'According to one researcher, the findings provide "beautiful confirmation" of standard theories to explain how structures in the Universe evolved over billions of years.'"

What standard theories? Dark matter does not exist, as least not as far as anyone (except astronomers with good imaginations) knows. There is a very nice (and complete!) standard model of physics, and dark matter holds no place.

Regular matter, that is simply dark - i.e. cold, and not emiting light, does not bother me. But making up particles no one has ever seen just because you don't understand what you are seing is fitting facts to the data.

Scientists often discuss new theories, etc, and in that context dark matter has it's place, but to claim it exists - as this story does - without being able to actually measure anything is quite silly and premature. If you don't understand something, say so, don't invent plausable explanations that have nothing supporting them except your lack of knowledge.

"We understand statistically what those galaxies are supposed to look like,"

So this map is based on what they assume the universe should look like. Then they use how its different to find where the dark matter might be. Doesnt sound 100% certain by any means, but its a nice picture.

Isn't that exactly what they did? They measured mass distributions through gravitational lensing and noted the places where there was more apparent mass then there should be. You can theorize that gravity works strangely at large scales, and inconsistently too, since they found clumps, but the simplest explanation that matches the observations is that there is something with mass that we can't see. It might be normal matter, but the fact that there's an enormous amount of it and it somehow avoids rubbing together and getting hot like all the other matter we know of is problematic. When galaxies collide it also seems to just keep on going while the normal matter slows down when it hits something going the other direction. Given those two observations (dark and appears not to interact other than gravitationally), a subatomic particle isn't so bad an explanation. It's not so far fetched either -- we know of other particles that have those properties. They're called neutrinos.

Actually astronomers do:). Most galaxies are "disk galaxies", i.e. lenticulars and spirals. Face-on (viewed from above), they look like a disk. So they should look like ellipsoids when viewed on the sky, due to their inclination. But this basic shape gets distorted when viewed through a lens (in this case, the lens is a massive object in front). The distortions are very small, so what astronomers do is that they measure the shape of as many galaxies as possible in a given region, and look for a statistical departure from the expected one.
There is no assumption on the Universe is supposed to look like in this map. The only assumption is that the General Relativity of gravitation is correct. So far, it has not been disproved.

This is what I don't get about dark matter, and this is just how I see it, and no one has ever given me a decent explanation. Why is it that scientists think that dark matter exists simply because the observed galaxies don't conform to Newton's Laws? Wouldn't a simpler solution be to take a step back and consider that, maybe, Newton's Laws are flawed? I am not trying to disprove dark matter, I certainly am no cosmologist, but it just seems odd that so much attention is given to dark matter, and very little is given to competing theories, such as MOND [wikipedia.org](Modified Newtonian Dynamics), that to me, at least, make more sense.

My basic point is, from a layman's perspective, dark matter just sounds like something physicists pulled out of thin air to explain something they don't understand. Your observations don't make sense? Well, throw in some dark matter and we're good!

Can someone explain to me why dark matter is the prevalent theory? Or perhaps why something like MOND is always ignored? As I said, I don't know what is right, but it just seems like a hack-job to me.

It's not completely true that MOND does not get any attention, there are very regularly publications in refereed journals about it, to prove it, disprove it, or try to make it better. Here is the problem as I see it:

The current accepted theory of gravitation, general relativity, works extremely well: it's predictive power has so far never be successfully challenged. Many have tried, noone has succeeded.

MOND had some success in explaining various observational puzzles, but has also some problems with others, as the wikipedia entry you link indicates.

MOND is an ad-hoc theory, just like dark matter is an ad-hoc solution

It is very hard to change your theoretical framework, much easier to add some stuff to the universe.

This explains to me why, right now, the current accepted paradigm is dark matter. While it is not satisfying, it is enough to explain both the rotation curves of disk galaxies, and the formation and evolution of the large scale structure of the Universe. I don't think many astronomers are satisfied with this current situation, and some are trying to resolve the issue, either explaining dark matter or getting rid of it. The others find with dark matter a framework where they can go on in exploring other scientific questions, like the evolution of galaxies, where you need to explain how their (normal) matter was assembled together, but also how this matter (gas) is made into stars, etc...

70% of the universe is made of some theoretical "substance" that hasn't shown up in several decades of particle physics observations??

I smell a Thomas Kuhn moment in the making. Or at least, a phlogiston moment.

Explaining the universe is hard. But saying stuff like "it's real", even implying that it is... when there's not even a working theory about it yet... is dangerous to the craft. When people get religious about stuff like string theory, it endangers science.

Close, dark matter is the "hack" we use to make our theories fit our obervations. If we had not observed "something" we would not need to invent the name "dark matter" to label it.

I fail to see how it is a "poor excuse" for anything, it's mearly a description of something we don't fully undersatnd but can indirectly observe and therfore label. Maybe our elegant theories will need to change to account for future observation but right now our notion of what we label as "dark matter" explains the observed anomolies better than any other concept, including the proposed modifications to gravitational theory.

For a historical perspective you just need to go back a hundered years to a time when scientists were having a similar debate about the existance and structure of atoms. Sure the model of atoms looking like "a pudding with razor blades stuck in it" fell by the wayside when it failed to explain all the observations. That is how science progresses, it's an evolution of ideas and analogies, not a static statement of "the truth".

Some skepticism is certainly in order. Since we currently have no way of independently confirming the existence of dark matter, we also have no way of distinguishing between two possible cases: one case is that dark matter corresponds to some real, physical material; the other is that the theory of gravity we're using is flawed. The fact that a better theory of gravity hasn't been produced doesn't mean that the current one is correct.

There are pretty strong parallels between dark matter and the infamous epicycles [wikipedia.org]. The case for epicycles was about as strong as that for dark matter: epicycles were a construction required to make the theory work, but there was no way to independently verify their existence, and they turned out to be essentially fictitious (assuming one doesn't take the position that they could be turned into a valid way of describing the solar system's orbital motion taking the Earth as center.)

The real problem is that there are no checks and balances here: by adjusting the mass distribution of dark matter, we can get whatever result we want, and there's nothing to either prove or disprove the proposed distribution. It's the ultimate hack, since it can be adjusted to suit every individual galaxy we observe.

Screwing around with the laws of gravity isn't any more elegant,

In the absence of independent evidence of dark matter, it would be more elegant if laws of gravity were discovered which explained the observations well without dark matter.

and there are plenty of plausible candidate particles for dark matter lying around in various extensions to the Standard Model.

That's a pretty weak position. It certainly doesn't do anything to counter the accusation that objects are being invented just to make the theory work.

Dark matter is a crutch of a theory with so many problems they had to invent an imaginary substance to explain them.

The history of science is filled with examples of new particles that were predicted — and discovered — on the basis of experimental discrepancies. You're going to have to do better than that.

The term "dark matter" originally referred to normal matter that we couldn't see because it wasn't lit up. Once this idea was proven inadequate, dark matter became something new and its definition was shaped solely by what the theorists needed it to be.

You say that like it's a bad thing. Theories that don't work are replaced by theories that do.

But even with these inventions, they are routinely surprised by what they find in the universe.

So? Nobody has claimed that we know everything about the universe. Dark matter and dark energy are features of our universe, but they don't explain everything about it.